The present invention relates to novel perovskite oxygen reduction electrode materials comprising partial copper substitution at the B-site of an ABO3 perovskite and methods for making and using same. Electrode materials in accordance with the invention find advantageous use in oxygen reducing electrochemical devices such as, for example, solid oxide fuel cells, oxygen separators, electrochemical sensors and the like.
As a background to the invention, electrochemical devices based on solid oxide electrolytes have received, and continue to receive, significant attention. For example, solid state oxygen separation devices have received significant attention for the separation of pure oxygen from air. In addition, electrochemical fuel cell devices are believed to have significant potential for use as power sources. Fuel cell devices are known and used for the direct production of electricity from standard fuel materials including fossil fuels, hydrogen, and the like by converting chemical energy of a fuel into electrical energy. Fuel cells typically include a porous fuel electrode (also referred to as the “anode”), a porous air electrode (also referred to as the “cathode”), and a solid or liquid electrolyte therebetween. In operation, gaseous fuel materials are contacted, typically as a continuous stream, with the anode of the fuel cell system, while an oxidizing gas, for example air or oxygen, is allowed to pass in contact with the cathode of the system. Electrical energy is produced by electrochemical combination of the fuel with the oxidant. Because the fuel cells convert the chemical energy of the fuel directly into electricity without the intermediate thermal and mechanical energy step, their efficiency can be substantially higher than that of conventional methods of power generation.
Solid oxide fuel cells (SOFCs) employing a dense ceramic electrolyte are currently considered as one of the most attractive technologies for electric power generation. In a typical SOFC, a solid electrolyte separates the porous metal-based anode from a porous metal or ceramic cathode. Due to its mechanical, electrical, chemical and thermal characteristics, yttria-stabilized zirconium oxide (YSZ) is currently the electrolyte material most commonly employed. At present, the anode in a typical SOFC is made of nickel-YSZ cermet, and the cathode is typically made of lanthanum manganites, lanthanum ferrites or lanthanum cobaltites. In such a fuel cell, an example of which is shown schematically in FIG. 1, the fuel flowing to the anode reacts with oxide ions to produce electrons and water. The oxygen reacts with the electrons on the cathode surface to form oxide ions that migrate through the electrolyte to the anode. The electrons flow from the anode through an external circuit and then to the cathode. The movement of oxygen ions through the electrolyte maintains overall electrical charge balance, and the flow of electrons in the external circuit provides useful power.
Because each individual electrochemical cell made of a single anode, a single electrolyte, and a single cathode generates an open circuit voltage of about one volt and each cell is subject to electrode activation polarization losses, electrical resistance losses, and ion mobility resistant losses which reduce its output to even lower voltages at a useful current, a fuel cell assembly comprising a plurality of fuel cell units electrically connected to each other to produce the desired voltage or current is required to generate commercially useful quantities of power.
SOFCs typically operate at high temperatures, such as, for example, 650-1000° C. This allows flexibility in fuel choice and results in suitable fuel-to-electricity and thermal efficiencies; however, high temperatures impose stringent requirements on the materials selection for other components of the fuel cell or fuel cell assembly. For example, it is well recognized that such high temperatures prevent the use of metallic materials in certain components and prevent the use of other materials that would otherwise be advantageous, but that are not stable at such temperatures. In contrast, lower operating temperatures, such as, for example, temperatures of 650° C. or less, would allow the use of high temperature steels as interconnect materials and would allow the use of other desirable materials in the system, which would significantly reduce the cost of fabrication and increase the reliability of SOFC stacks.
Notwithstanding the advantages of lower operating temperatures, difficulties have been encountered in attempts to design SOFC systems that will operate efficiently at relatively lower temperatures, such as, for example, temperatures of about 650° C. or lower. For example, the materials typically used as electrodes in SOFC systems are perovskite materials that do not have suitable electrical properties at such lower temperatures. In particular, attempts to design SOFC systems that operate at lower temperatures have been unsuccessful due to the greatly reduced performance of cell components, primarily the cathode.
Current SOFC cathode development has focused on lanthanum strontium ferrite (LSF). While a substantial improvement over the older lanthanum strontium manganite in the 700° C. to 900° C. range, the performance of an SOFC would benefit from further improvements in the properties of the cathode over those exhibited by LSF.
In view of the above background, it is apparent that there is a continuing need for further developments in the field of SOFC technology. In particular, there is a need for further advancement in the development of alternative cathode materials having suitable properties for use in advanced SOFC designs including, for example, materials featuring good electrode kinetics for oxygen reduction at relatively low temperatures and robust physical and chemical durability. There is also a need for further advancement in the development of other alternative electrochemical devices that actively reduce oxygen in response to an electrical bias, such as, for example, oxygen separation devices, electrochemical sensors and the like. The present invention addresses these needs, and further provides related advantages.